1. Technical Field
The present invention generally relates to wireless communication networks, and particularly relates to packet retransmission processing and packet combining in such networks.
2. Background
Noisy communication channels, such as those present in wireless communication networks, inevitably cause communication errors. Channel coding schemes, wherein underlying information bits are interleaved and/or encoded for transmission, offer significant error protection and error detection capabilities, based on corresponding decoding processing at targeted receivers. Even with such schemes, however, decoding errors may arise at the receiver. Numerous retransmission techniques have been devised to deal with decoding failures, wherein receivers request packet retransmissions responsive to detecting packet decoding failures, e.g., detected CRC (cyclic redundancy check) failures.
In particular, one or more HARQ (hybrid automatic repeat request) retransmission schemes are integral to several current and developing wireless communication standards, including HSDPA (High Speed Downlink Packet Access) services and HSUPA (High Speed Uplink Packet Access) services in WCDMA (Wireless Code Division Multiple Access) networks. HARQ retransmission schemes commonly use either CC (Chase Combining) or IR (Incremental Redundancy) techniques for failed packet retransmission.
For example, in response to receiving a retransmission request for a given failed packet, a CC transmitter retransmits the failed packet. Correspondingly, the requesting receiver combines the repeated copy of the failed packet with its original copy, for improved decoding accuracy. In contrast, an IR transmitter retransmits a different set of parity bits related to a failed packet for which a retransmission request was received. The requesting receiver uses both the old and the new parity bits to detect/correct decoding errors in a subsequent decoding attempt for the failed packet. Because CC allows the receiver to combine the repeated packet with the originally received copy of that packet, its memory requirements are generally less than that of IR, where the receiver must retain the old and the new parity bits for re-decoding of the failed packet.
Despite its memory advantages for the receiver, CC-based retransmission schemes do not allow fine granularity with respect to changing the (en)coding rate used for retransmitted packets. For example, assume that a failed packet originally was transmitted using ½ rate encoding. Retransmission of that packet effectively reduces the coding rate for that packet to ¼. That reduction in coding rate represents transmission inefficiency to the extent that a less aggressive coding rate, ⅓, for example, would have sufficed.
Further, CC-based retransmission schemes can be particularly inefficient in select repeat contexts. For example, with selective repeat HARQ, consider the case where two packets A1 and A2 are broadcast to two receivers R1 and R2. It is possible, for example, that receiver R1 receives packet A1 incorrectly and receiver R2 receives packet A2 incorrectly. In that case, the transmitter has to resend the A1 packet so that receiver R1 can correctly decode the A1 packet, and has to resend the A2 packet so that the receiver R2 can correctly decode the A2 packet. Put simply, selective repeat retransmission schemes require a separate retransmission for each packet that is incorrectly received at any receiver within a given group of receivers.